Water diverted from the Mississippi River spills through a bay in the Morganza Spillway. A steel, 10-ton floodgate was slowly raised Saturday for the first time in nearly four decades, unleashing a torrent of water from the Mississippi River, away from heavily populated areas downstream.

New Orleans — One hour. That’s all it took for the farmland located beside the Morganza floodwall, untouched for 38 years, to turn into a great lake.

Water from the Mississippi rolled gently but quickly through the natural landscape of trees, grazing land, and swamp late Saturday afternoon, the result of a decision by the US Army Corps of Engineers to open the historic hydraulic system in order to lower river levels cascading towards Baton Rouge and New Orleans.

It will take three days for the water to flow 20 miles south into the Atchafalaya Basin, which drains into the Gulf of Mexico. Along the way, the water’s passage will affect up to 25,000 people living in communities, from small towns populated by just a few hundred to Morgan City, central to the area’s oil and seafood production, that is home to 12,000 people. Sandbagging of roads, interstate highways, and homes in those areas started early last week.

The Morganza is the second floodway opened in Louisiana to mitigate the surging floodwaters driving southward down the Mississippi River over the past three weeks that has displaced hundreds and continues to cover homes and farmland with water.

Mike Strack, chief of emergency management for the Corps’ New Orleans division, says that even though the Corps opened a single bay, there are 124 remaining. The opening will continue to be slow to give wildlife, including black bears, deer and turtles, time to find shelter. However, the flow rate will rise, from 10,000 cubic feet per second (cfs) to about 125,000 cfs, Corps officials said. That represents less than a quarter of the structure’s full capacity of 600,000 cfs.

Last Sunday, the Corps opened all 350 bays at the Bonnet Carre, located closer to New Orleans, releasing water into Lake Pontchartrain and eventually the Gulf of Mexico.

There is still uncertainty about how much water to let through and for what period of time.

Officials say they are aiming for at least three weeks and will involve the opening of at least 15 bays. But as forecasts from the National Weather Service change, it may be longer.

Water is expected to crest at the Morganza in about ten days. How far the river levels rise will determine how much will end up being diverted through the floodway.

The Morganza was built in 1954 and used only one other time before, in 1973. Still there remain uncertainties: Will the Corps be able to close the steel doors of the floodway gates three weeks after they are open? Is using only a quarter of the Morganza’s capacity enough to prevent flooding in New Orleans, where water rests less than three feet from the top of levees?

John Barry, author of “Rising Tide,” considered the definitive history of the 1927 flood, the most disastrous flood in US history to date that displaced over 600,000 people and flooded 26,000 square miles, says the actions taken by the Corps since that incident produced “a very good comprehensive system that includes all sorts of elements including, by a wide margin, the strongest levees in the U.S.”

The agricultural use of the floodplain is understandable because it “is too valuable to just leave fallow,” Mr. Barry says. But opening the Morganza was a calculated risk work taking, he adds. River levels in New Orleans, for example, are expected to crest at 19.5 feet, just inches from the top of the levees there.

Barry says that possibility could lead to a “massive levee breach” where there would be “Niagara Falls or more pouring of the river for an extended period of time.”

“Once that breach opens, that can’t be closed,” he says, which would cause worse flooding than the city experienced following Hurricane Katrina.

The moment the gate opened Saturday afternoon, there was a sense of mourning but also relief.

Minutes earlier, a local radio station played gospel music from the late Mahalia Jackson and also Randy Newman’s “Louisiana 1927,” a musical elegy with a chorus – “they’re gonna wash us away” – that likely articulated what many downstream were feeling.

Fish could be seen jumping into the air following the first blast of water. Although the road leading across the floodway was temporarily closed to the public, the levees along the floodway were lined with people who showed up, some with binoculars in hand, to get a look.

“It’s making people feel comfortable again. The water has to go somewhere,” says Ernest Newman, a bus driver from Morganza who walked to a viewing point with his family. “I’m sorry for the farmers.”

Herman Kleinke and his daughter Evelyn, from Columbia, Miss., were already in the area and decided to drive to Morganza to witness an event Ms. Kleinke called “historical.”

“This is something we can tell grandkids,” she says.

Two hours to the east in New Orleans, the anxiety the town had been experiencing since the river started to rise, was starting to subside. After all, this is a town that is surrounded by water that is largely out of sight and often out of mind.

Climbing the levees to watch river levels rise became a common activity these last few days, as was answering phone calls from out-of-town friends and family offering refuge in case the streets started to fill with water.

Larry Lovell, an advertising executive who directed the tourism media center in the immediate wake of Hurricane Katrina, went for a run atop the levees in his neighborhood Saturday morning and says that even though the city had previously witnessed water levels at such a height, it did give him pause. Daily updates about rising river levels had created “Katrina flashback” for him, and for many others here.

'Some sense of relief'

Knowing the Morganza had opened “gives us some sense of relief,” Mr. Lovell says, that “ensures we won’t get to those levels that would get to that level of catastrophic failure.”

“We know we can deal with this, but not making us deal with the highest level of risk in the shortest period of time gives me a level of comfort,” he says.

Besides its obvious use for diverting disaster from the city’s two largest metropolitan areas, the Morganza’s opening is also significant from an engineering perspective as it represents the first time that all three floodways built following the 1927 flood were in operation at the same time. Besides the Bonnet Carre, the Birds Point floodway in Missouri was opened earlier this month.

For those involved in flood management policy at the federal and state level, what happens over the new few weeks is critical in determining the effectiveness of the floodways.

Shana Udvardi, a director at American Rivers, an advocacy organization in Washington, says that the Corps should invest in developing alternate systems, such as release valves along the river and its tributaries that could serve dual purposes: Help farmers store water on wetlands for agricultural use while slowing down water during flooding season.

“Instead of waiting for the flood to come to us, hopefully we can get at reducing flood risk,” Ms. Udvardi says. “We are really relying on outdated approaches. The levees should really be the last line of defense than the only one.”